Polyalphaolefins (PAOs) of different viscosity grades are known to be useful in synthetic and semi-synthetic industrial oil and grease formulations. See, for instance, Chapters 22 and 23 in Rudnick et al., “Synthetic Lubricants and High-Performance Functional Fluids”, 2nd Ed. Marcel Dekker, Inc., N.Y. (1999). Compared to the conventional mineral oil-based products, these PAO-based products have excellent viscometrics, high and low temperature performance and energy efficiency under routine conditions and ordinary replacement schedules.
The viscosity-temperature relationship of a lubricating oil is one of the critical criteria, which must be considered when selecting a lubricant for a particular application. Viscosity Index (VI) is an empirical, unitless number which indicates the rate of change in the viscosity of an oil within a given temperature range. Fluids exhibiting a relatively large change in viscosity with temperature are said to have a low viscosity index. A low VI oil, for example, will thin out at elevated temperatures faster than a high VI oil. Usually, the high VI oil is more desirable because it has higher viscosity at higher temperature, which translates into better or thicker lubrication films and better protection of the contacting machine elements. In another aspect, as the oil operating temperature decreases, the viscosity of a high VI oil will not increase as much as the viscosity of a low VI oil. This is advantageous because the excessively high viscosity of the low VI oil will decrease the efficiency of the operating machine. Thus a high VI oil has performance advantages in both high and low temperature operation. VI is determined according to ASTM method D 2270-93 [1998]. VI is related to kinematic viscosities measured at 40° C. and 100° C. using ASTM Method D 445-01.
PAOs comprise a class of hydrocarbons manufactured by the catalytic oligomerization (polymerization to low molecular weight products) of linear α-olefins typically ranging from 1-hexene to 1-octadecene, more typically from 1-octene to 1-dodecene, with 1-decene as the most common and often preferred material. Examples of these fluids are described, by way of example, in U.S. Pat. Nos. 6,824,671 and 4,827,073, although polymers of lower olefins such as ethylene and propylene may also be used, especially copolymers of ethylene with higher olefins, as described in U.S. Pat. Nos. 4,956,122 or 4,990,709 and the patents referred to therein.
High viscosity index polyalphaolefin (HVI-PAO) are prepared by, for instance, polymerization of alpha-olefins using reduced metal oxide catalysts (e.g., chromium) such as described in U.S. Pat. Nos. 4,827,064; 4,827,073; 4,990,771; 5,012,020; and 5,264,642. These HVI-PAOs are characterized by having a high viscosity index (VI) and one or more of the following characteristics: a branch ratio of less than 0.19, a weight average molecular weight of between 300 and 45,000, a number average molecular weight of between 300 and 18,000, a molecular weight distribution of between 1 and 5, and pour point below −15° C. Measured in carbon number, these molecules range from C30 to C1300. Viscosities of the HVI-PAO oligomers measured at 100° C. range from 3 centistokes (“cSt”) to 15,000 cSt. These HVI-PAOs have been used as base stocks since their commercial production and are commercially available, such as for instance SpectraSyn Ultra™ fluid, from ExxonMobil Chemical Co.
Another advantageous property of these HVI-PAOs is that, while lower molecular weight unsaturated oligomers are typically and preferably hydrogenated to produce thermally and oxidatively stable materials, higher molecular weight unsaturated HVI-PAO oligomers useful as lubricant are sufficiently thermally and oxidatively stable to be utilized without hydrogenation and, optionally, may be so employed.
HVI-PAO materials have been used for formulating oils for internal combustion engines. By way of example, WO 00/58423 teaches high performance oil comprising a first and second polymer of differing molecular weights dissolved in a base stock of low viscosity. The first polymer is a high viscoelastic polymer, preferably an HVI-PAO. The base stock used generally has a viscosity of below 10 cSt at 100° C. The HVI-PAO is “normally present in relatively small amounts”, e.g., 0.1 to about 25 wt % in the total finished product. Also included in the finished product is a polymeric thickener, normally based on block copolymers produced by the anionic polymerization of unsaturated monomers including styrene, butadiene, and isoprene. A “conventional” additive package, containing dispersant, detergents, anti-wear, or antioxidants such as phenolic and/or amine type antioxidants is also added.
See also U.S. Pat. Nos. 4,180,575; 4,827,064; 4,827,073; 4,912,272; 4,990,771; 5,012,020; 5,264,642; 6,087,307; 6,180,575; WO 03/09136; WO 2003071369A; U.S. Patent Application No. 2005/0059563; and Lubrication Engineers, 55/8, 45 (1999).
Industrial gear oils have to meet the following requirements: excellent resistance to aging and oxidation, low foaming tendency, good load-carrying capacity, neutrality toward the materials involved (ferrous and nonferrous metals, seals, paints), suitability for high and/or low temperatures, and good viscosity-temperature behavior; gear greases, in contrast, are required to ensure the following: good adhesion, low oil separation, low starting torques, compatibility with synthetic materials, and noise dampening (c.f., Rudnick et al., supra). Heretofore, a universal gear lubricant meeting all these requirements is not, as far as the present inventors are aware, commercially available. This requires that lubricant manufacturers develop different types of formulations with properties satisfying individual operating needs for each application.
Industry is developing a very high viscosity index (VI) finished gear lubricants comprising Group IV and Group V base fluids. Many of these next generation gear lubricants utilize chrome-silica catalyst derived 150 cSt PAO. This very high viscosity index Group TV base oil, when combined with very low viscosity base oil components (PAO 2 and a polar Group V base stock) has displayed distinct efficiency and VI advantages over prior art synthetic gear lubricants. It is understood that the high VI and wide bi-modal viscosity distribution of the components contributes significantly to the fluid performance advantages.
One area for improvement to this bi-modal base fluid system is the relative shear instability of the high viscosity Group IV base stock—150 cSt PAO. Current formulation options using 150 cSt SuperSyn™ utilize 40-50% of this component in the finished fluid. Due to the rather wide molecular weight distribution of the existing chrome-silica derived SuperSyn base stocks (cs-SuperSyn), some shear instability is observed in traditional lubricant shear tests (KRL 20 hr Bearing Shear Test/CEC L-45-A-99). This shear instability may lead to overall permanent viscous losses of the finished fluid.
There is a need to improve shear stability in bi-modal lubricant formulations. Accordingly, this inventions satisfies that need.